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Edifice Stability Assessment of Lassen Peak, Lassen Volcanic National Park, California
Geological Sciences and Engineering
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Lassen Peak (one of the largest active lava domes on Earth) is the southernmost active volcano in the Cascade Range, located in Lassen Volcanic National Park (LVNP), California. Large scale slope failures have been identified in other areas of LVNP, but not at Lassen Peak, which may be prone to major instabilities, as eruptions within the past 100 years and its relatively active tectonic setting may have weakened its dome and edifice. This edifice stability assessment of Lassen Peak focused on locating hydrothermally altered and / or structurally weakened zones, quantifying geotechnical properties of the rock mass, and modeling potential failure surfaces within the edifice using limit equilibrium slope stability techniques. Results of the assessment indicate that the northeast and northwest flanks are the most likely areas for catastrophic circular (global) failures, though these potential instabilities would likely need to by triggered by a combination of earthquake loading and elevated fluid (groundwater, gas, or magma) pressures. Understanding fluid pressures within active volcanoes is a complex issue, where some of the destabilizing forces are functions of rock mass permeability, the depths of intrusions, magma degassing, and magma viscosity. Water table sensitivity analyses were performed to estimate equivalent fluid pressures and indirectly model gas and magma pressures. Due to the highly cohesive rock mass, the circular failures may be deep-seated, potentially displacing rock volumes up to 4 cubic km from the northeast flank and 2 cubic km from the northwest flank. Based on field observations and trends in structural data, a large-scale planar failure (displacing about 0.1 cubic km) is possible within the southeast flank but would require triggering by elevated joint fluid pressures and/or seismic loading.